Solar energy technology aims at providing economically competitive and environmentally friendly power for a variety of commercial applications. The efficiency of the conversion of solar energy into utilizable heat or electric power depends to a great extent on the light brightness achieved on the entrance surface of a solar energy receiver as well as on reflection, shadow and other losses, and is dependent on the available solar energy concentration.
Phase-space conversion and thermodynamic considerations place a theoretical limit on the concentration of sunlight that ally optical device can achieve (R. Winston et al. Approaching the irradiance of the surface of the sun, Solar Thermal Technology, Proc. 4th Intern. Symposium, Santa Fe, N.M., pp. 579-587, 1988). This limitation is expressed by the equation ##EQU1## where C.sub.max is the maximum attainable concentration, n is the refractive index of a target surface and .alpha. is the half-angle of incidence, of the sunlight. This theoretical limit is derived under the assumption that the target area is large enough to collect all of the concentrated light.
The concentrations attainable in practice by conventional imaging devices fall far short of this limit owing to aberrations. For example, a parabolic mirror produces a perfect image on axis but the image blurs and broadens off axis.
By dispensing with image-forming requirements ill applications where no image is required, much higher concentration can be achieved. Non-imaging optics is known to allow light concentration close to the thermodynamic limit and, therefore, this technology is frequently used in powerful concentrators.
A practical approach for high concentrations of sunlight usually utilizes a two-stage optical system that incorporates a first-stage (primary) imaging concentrator which redirects incident solar radiation towards the focal point, and a second-stage (secondary) non-imaging concentrator which directs the concentrated solar radiation onto the solar absorber of a solar receiver. The secondary concentrator is placed close to the focus of the primary one and provides acceptance of all redirected solar energy as well as high brightness at the entrance of the receiver. The overall concentration for a two-stage system is a product of the concentration of the primary concentrator with that of the non-imaging concentrator: ##EQU2## where C is an achievable concentration factor, C.sub.p is a concentration factor of a primary concentrator and .alpha. and .beta. are the maximum angles of distribution of the incoming and outgoing light.
The size and overall performance of the system are greatly affected by the nature of the primary concentrator. This imaging, first-stage concentrator may often be in a form of a parabolic or spherical mirror. The concentration provided by a parabolic dish may be calculated as follows: ##EQU3## where C.sub.p is the concentration factor for a parabolic dish, .alpha. is the maximum angle of distribution of incoming light, r is the radius of the parabolic dish and h is the so-called focal distance, i.e. the distance from the dish to the focal plane. The concentration usually achieved by a parabolic dish is less than 25% of a thermodynamic limit.
In an effort to concentrate the incoming solar radiation close to the thermodynamic limit, attempts have been made to use an imaging reflective telescope, e.g. a Cassegrain telescope, as a primary concentrator (W. Zittel, "Design Studies for Solar Pumped Lasers", DFVLR-FB 87-39, Stuttgart, 1987). However, such a telescope formed by a parabolic primary mirror and a hyperbolic secondary mirror has very low aberrations only for a very narrow acceptance angle. Therefore, to provide a high-power concentration, the telescope has to track the sun, which is practically impossible for this kind of system where the size of a primary reflective area may be of the order of tens or even hundreds of thousands of square meters.
For stationary receivers, the Fresnel reflector is often a primary concentrator of choice because of its construction and tracking simplicity (M. Epstein, "Central receiver facility at the Weizmann Institute of Science", Solar thermal central receiver systems, Proc. III Intern. Workshop, Springer-Verlag, Berlin, FRG, pp. 187-197, 21986. M. Epstein, "Beam quality and tracking accuracy results of the Weizmann Institute of Science Heliostats", Proc. 4th. Intern. Symp. on Research, Development and Applications of Solar Thermal Technology, New York, pp. 108-111, 1990).
There are different types of Fresnel reflectors. Two-dimensional (2-D) Fresnel reflectors with focal lines have been developed for use in commercial plants; three-dimensional (3-D) Fresnel reflectors with focal points, usually called heliostats field, arc used in conjunction with central solar receivers and solar towers, particularly in the megawatts scale systems, as they can operate at higher power fluxes and temperatures, thus allowing to achieve high conversion efficiencies. The heliostats field consists of a plurality of computer controlled mirrors which redirect solar radiation towards a secondary concentrator located in the region of the focal points usually located on a central solar tower, and followed by a volumetric central receiver. The concentration factor of the heliostats field may be calculated by the equation ##EQU4## where C.sub.f is the concentration factor of the heliostats field, C.sub.p is the concentration factor of an individual constituent parabolic or spherical mirror of the field, h is the focal length of the heliostats field and r is the field radius. Because of shadowing effects, even when the sun remains in zenith, and because the area of the redirected solar radiation is smaller than the reflective area of the heliostats, and the aggregate area of the heliostats is smaller than the gross area of the heliostats field, the available concentration of the heliostats field is less than that of a parabolic dish, and usually does not exceed 21%. It is clear from the above equations that the larger the focal distance of the concentrator or the larger the h/r ratio, the higher the concentration achievable. Thus, for better overall concentration the focal length of the heliostats field, which actually determines the height of the solar tower, should be as large as possible.
In a 100 MW scale solar system the height of a solar tower has to be 100 meters and more. Therefore, the secondary concentrator and the associated central solar receiver as well as some components of the energy conversion systems must all be installed at the top of the tower. This requirement poses difficult and expensive engineering problems which are aggravated by shadowing problems arising out of the fact that the solar light reaches the secondary concentrator from below. The focal distances of heliostats often exceed 300 meters for a solar field with a high solar tower which leads to significant aberrations and loss of concentration (L. L. Vant-Hull, M. E. Izogon and C. L. Pitman, "Results of a heliostat field: receiver analysis for Solar Two", Proceedings of the ACME International Solar Energy Conference, Washington, D.C. pp. 2243-2251, May 1993).
To sum up, high power solar energy plants with a heliostats field concentration system and a central solar receiver on top of a high tower, optionally in association with a secondary concentrator, poses serious problems of design and efficacy of primary concentration.
These problems have already been acknowledged in the past and in the attempt to solve them a so-called "tower reflector" concept has been proposed (A. Rabl, "Technical Note. Tower reflector for solar power plant", Solar Energy, Vol. 18, pp. 269-271, 1976). In accordance with this concept, a solar energy plant comprising a solar receiver and a heliostats field installed on a base plane and having a focal point above said base plane is provided with an additional, flat Fresnel reflector mounted on a solar tower close to the focal point, whereby concentrated solar radiation reflected by the additional reflector is redirected onto the solar receiver placed close to the base plane. To improve the radiation concentration achieved by the system, a compound parabolic concentrator placed in the vicinity of the receiver is employed. Thus, due to the use of the tower reflector, the solar receiver and any associated equipment can be installed close to the base plane rather than being mounted on top of a high tower.
However, as acknowledged in Rabl's disclosure, there exists a serious problem connected with a necessity to avoid overheating of the tower reflector which is to be exposed to the concentrated solar light of 100 suns or even more. With an ordinary construction of the reflector, based on metallic layers, a significant amount of this energy would have been absorbed by the reflector requiring an intensive cooling thereof, which is quite difficult and onerous at high altitudes of the tower. In order to solve the overheating problem, Rabl suggests that the elements of the tower Fresnel reflector be in the form of rectangular prisms with total internal reflection. In such a design the tower reflector of the solar energy plant has to be of relatively large dimensions anti have an extremely large mass. Moreover, with the tower reflector in the form of the flat Fresnel reflector, shadowing and blocking effects will take place causing deterioration of primary concentration, losses of solar radiation and, consequently, a rather low conversion efficiency of the solar plant. Finally, the costs of such an arrangement would be prohibitive. All these disadvantages render the construction proposed by Rabl practically inapplicable anti may explain why, until now, the tower reflector concept has not found its use.
It is the object of the present invention to provide a highly efficient solar energy plant with a tower reflector in which the above disadvantages are avoided.